1. Field of the Invention
The present invention relates generally to an internal combustion engine and more specifically to a multi-cylinder internal which has more than one inlet valve per combustion chamber and which is equipped with a variable valve timing control device which controls at least one of the inlet valves.
2. Description of the Prior Art
FIGS. 1 to 5 show an internal combustion engine disclosed in Japanese Patent First Provisional Publication No. 58-25537(1983) which is equipped with a dual induction system and two inlet valves per combustion chamber.
This engine is arranged so that the charge inducted through the first or primary induction system 1 is delivered into each cylinder through a first or primary inlet valve 2 in a manner which promotes a swirling flow pattern in the combustion chamber. The primary induction system 1 is designed to function efficiently during low engine speed modes of operation (see trace A in FIG. 5). This ensures stable operation during idling and the like.
The fresh charge inducted through the secondary induction arrangement 4 is introduced into the combustion chambers via secondary inlet valves 6. The port arrangements 8 associated with each of the secondary inlet valves 6 are larger in cross-section than those associated with the corresponding primary inlet valves 2 and are arranged to provide good charging efficiency rather than swirl.
Each cylinder is provided with a single exhaust valve 10 which is located as shown in FIG. 1, opposite the primary inlet valve 2.
Each of the exhaust and the inlet is operated by a rocker arm arrangement which includes a mechansim 12 which permits each valve to be selectively enabled or disabled.
Under low load-low speed operation all of the valves are disabled while under high load-low speed only the secondary inlet valves 2 are rendered inoperative.
FIGS. 3 and 4 show the valve lift control mechanism 12 which enables the selective disablement.
This mechanism includes a fork-like stopper arrangement 14 which is selectively movable under the influence of hydraulic pressure supplied to a control chamber 16. When it is desired to disable the valve associated therewith the hydraulic pressure in the control chamber 16 is relieved and the stopper member 14 permitted to move back away from a valve tappet 18 under the influence of a return spring 20. This retractive movement unlocks a tappet arrangement 18 and allows a plunger 22 thereof to reciprocate in and out of the main body 24 of the device and thus prevent the motion of the rocker arm 26 from being transmitted to the associated valve. The spring 20 of the tappet arrangement is weaker than the valve spring and thus maintains contact between the tip of the plunger 22 and the top of the valve stem without inducing any lift.
When it is desired to re-enable the disabled valve, hydraulic pressure is supplied to the control chambers 16 through a passage structure which includes an elongate coaxial bore 28 formed in the rocker arm shaft 30 and a branch runner 32 formed in the rocker arms per se. This drives the stopper forward into a position wherein it locks the plunger in position in the main body 24.
Fuel is supplied to the engine exclusively through the primary induction system 1. However, due to the on/off nature of the control which occurs with the enablement/disablement of the valves, in order to adequately control the A/F ratio of the air-fuel mixture actually fed to the combustion chambers it is necessary to use first and second fuel injectors 34, 35 in the SPI (single point injection) arrangement disposed upstream of the primary throttle valve 36. For example, upon enablement or disablement of the secondary inlet valves 6 the sudden change in induction volume is controlled by initiating or terminating the operation of one of the fuel injectors. However, this induces the drawback that the fuel supply system becomes complex and expensive.
With this type of engine system the primary induction system 1 is designed to function efficiently at low engine speeds wherein combustion stablity tends to be a problem while the secondary system 4 is designed to enable the production of large amount of power during high speed operation. If the secondary system 4 is not designed particularly for use at high engine speed it becomes impossible to develop the required amount of power under such conditions.
Thus, because the primary induction system 1 must be tailored to the needs of low speed operation and the secondary one to the needs of high engine speed operation a further drawback is encounted in that, as graphically illustrated in FIG. 5, during the transition between low and high speed operation a "flat spot" wherein the torque generation tends to fall off in the 2,500-4000 RPM region is produced. This deteriorates engine performance in this region.